Kotlin Design Patterns and Best Practices/

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Decorator Design Pattern

Decorator Design Pattern

Learn how to implement the Decorator design pattern and the Elvis operator in Kotlin.

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In Prototype design pattern, we can create instances of classes with slightly (or not so slightly) different data. But, this raises a question:

What if we want to create a set of classes that all have slightly different behavior?

Well, since functions in Kotlin are first-class citizens, we could use the Prototype design pattern to achieve this aim. After all, creating a set of classes with slightly different behavior is what JavaScript does successfully. But there is another approach to the same problem. After all, design patterns are all about approaches.

By implementing the Decorator design pattern, we allow the users of our code to specify the abilities they want to add.

Enhancing a class

Let’s say that we have a rather simple class that registers all of the captains in the Star Trek universe along with their vessels:

open class StarTrekRepository {
    private val starshipCaptains = mutableMapOf("USS
       Enterprise" to "Jean-Luc Picard")
    open fun getCaptain(starshipName: String): String {
    return starshipCaptains[starshipName] ?: "Unknown"
    }
    open fun addCaptain(starshipName: String, captainName:
    String) {
       starshipCaptains[starshipName] = captainName
     }
}
An open Star Trek repository class with methods for getting and adding starship captains

One day, your captain—sorry, scrum master—comes to you with an urgent requirement. From now on, every time someone ...